Game of Thrones: Which Dragon Becomes a White Walker?
A major element of House of the Dragon's plot is the prophecy of Aegon the Conqueror, or Aegon's Dream, which in large part motivates actions of King Viserys Targaryen and motivates Queen Rhaenyra as well.
That prophecy tells of the coming threat from the North, from beyond the Wall.
"Aegon saw absolute darkness riding on those winds. And whatever dwells within will destroy the world of the living", and declares that "a Targaryen must be seated on the Iron Throne" to meet that threat.
The showrunners have confirmed that the idea of the prophecy came from George Martin himself.
But ironically, in Game of Thrones the prophecy fails to come true (as George Martin's fantasy saga is not yet complete, we have no idea whether it will come true according to his own vision, from which showrunners of Game of Thrones deviated frequently in later seasons).
There is no Targaryen on the Iron Throne when the decisive battle with the Others, also known as the White Walkers, occurs. And worse still, that battle occurs because one of Daenerys' dragons becomes the mightiest creature of the Night King's undead host.
His flames easily blast through the Wall, allowing the White Walker swarm to pour south, into the lands of the living.
But what dragon meets this fate?
Viserion is the dragon who gets killed by the Night King with a magical ice spear, when Dany flew north to rescue Jon Snow. Perhaps naming a dragon after her abusive and inept brother, Viserys (not to be mistaken for the above-mentioned King Viserys) attracted bad luck.
Unfortunately, dragons to prove to be as susceptible to the Night King's power, which allowed him to reanimate and command the dead, as all other creatures.
And so, instead of repelling the threat of the White Walkers, the Targaryen dynasty managed to provide them with their most effective weapon and quite possibly the only easy method to breach the Wall. Whether the prophecy was just wrong, or whether Game of Thrones' showrunners changed the White Walkers' plotline as they pleased, instead, we're unlikely to know any time soon.